Nope; the extension is OPTIONAL.

http://perldoc.perl.org/perlrun.html:

The extension, if supplied (emphasis added), is used to....

And though this is already discussed (below), the examples (in the 5.12 doc) of using -p, -i and -e together are constructed this way:

perl -pi -e ...

except where the -p is separated by a space from a -i (and I didn't find an explicit explanation of that, either).